Principle of Resonance Raman Spectroscopy

In normal Raman spectroscopy, the exciting frequency lies in the region where the compound has no electronic absorption band. In resonance Raman spectroscopy, the exciting frequency falls within the electronic band (Sec. 1.2). In the gaseous phase, this tends to cause resonance fluorescence since the rotational-vibrational levels are discrete. In the liquid and solid states, however, these levels are no longer discrete because of molecular collisions and/or intermolecular interactions. If such a broad vibronic bands is excited, it tends to give resonance Raman rather than resonance fluorescence spectra [101,102]. 

Resonance Raman spectroscopy is particularly suited to the study of biological macromolecules such as heme proteins because only a dilute solution (biological condition) is needed to observe the spectrum and only vibrations localized within the chromophoric group are enhanced when the exciting frequency approaches that of the relevant chromophore. This selectivity is highly important in studying the theoretical relationship between the electronic transition and the vibrations to be resonance-enhanced. 

The origin of resonance Raman enhancement is explained in terms of Eq. 1.201. In normal Raman spectroscopy, Vo is chosen in the region that is far from the electronic absorption. Then, v vq, and a.p is independent of the exciting frequency vq. In resonance Raman spectroscopy, the denominator, Vq, becomes very small as vq approaches v. Thus, the first term in the square brackets of Eq. 1.201 dominates all other terms and results in striking enhancement of Raman lines. However, Eq. 1.201 cannot account for the selectivity of resonance Raman enhancement since it is not specific about the states of the molecule. Albrecht [103] derived a more specific equation for the initial and final states of resonance Raman scattering by 

For the A term to be nonzero, the g electronic transition must be allowed, and the product of the integrals (/ v) v j) (Franck-Condon factor) must be nonzero. The latter condition is satisfied if the equilibrium position is shifted by a transition (A 0 as shown in Fig. 1.30). Totally symmetric vibrations tend to be resonance-enhanced via the A term since they tend to shift the equilibrium position upon electronic excitation. If the equilibrium position is not shifted by the g e transition, the A term becomes zero since either one of the integrals, (/1 v) or (v 17), becomes zero. This situation tends to occur for nontotally symmetric vibrations. 

In contrast to the A term, the B term resonance requires at least one more electronic excited state(s), which must be mixed with the e state via normal vibration, Qa. Namely, the integral (e I I s ) must be nonzero. The -term resonance is significant only when these two excited states are closely located so that the denominator, E — E, is small. Other requirements are that the and g s 

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